This application is based on Japanese Patent Application No. 9-75187, filed Mar. 27, 1997, the content of which is incorporated herein by reference.
The present invention relates to an analog signal transmission circuit, and in particular to an analog signal transmission circuit in which a DC offset current is cut by polar capacitors.
In general, an analog signal transmission circuit provided in a modem or the like incorporates a drive circuit which has a transformer for transforming power and insulating the analog signal transmission circuit from an external circuit. The transformer is driven by a single-end type amplifier or a push-pull type amplifier (hereinafter referred to as a differential amplifier).
FIG. 1 shows a conventional analog signal transmission circuit in which the transformer is driven by a differential amplifier 1. As seen from FIG. 1, the amplifier 1 is a differential input/output type amplifier and has a positive output terminal 2 and a negative output terminal 3. Signals are supplied from the output terminals 2 and 3 to the transformer 6 through output resistors 4 and 5, respectively. The values of the output resistors 4 and 5 are determined so as to satisfy the impedance matching with the characteristic impedance of a communication circuit connected to secondary windings of the transformer 6. The output terminals 2 and 3 of the amplifier 1 are connected to the transformer 6 in a DC manner. In general, a DC voltage is superimposed on the output of the amplifier 1. The voltage between the output terminals 2 and 3 includes an error voltage referred to as "offset voltage". In the circuit shown in FIG. 1, a DC offset current I flows due to the offset voltage as shown in FIG. 2. The offset current I flows through windings of the transformer 6. The offset voltage at the positive output terminal 2 is higher than that of the negative output terminal 3. Thus, the current flowing through the windings of the transformer 6 has the waveform shown in FIG. 3. The offset current I flows from the positive output terminal 2 to the negative output terminal 3 as indicated by arrows in FIG. 2.
If the offset voltage at the negative output terminal 3 is higher than that at the positive output terminal 2, the offset current I flows from the negative output terminal 3 to the positive output terminal 2.
FIG. 4 shows a conventional analog signal transmission circuit in which no offset current I flows through the windings of the transformer 6. The circuit has a capacitor 7 for cutting a direct current. Thus, the capacitor 7 prevents the offset current I from flowing through the windings of the transformer 6. The capacitor 7 is a non-polar one as in most analog signal transmission circuits. This is because which potential is higher, the potential at the positive output terminal 2 or the potential at the negative output terminal 3, depends on each analog signal transmission circuit.
Generally, a circuit for processing low-frequency signals has a capacitor which has a large capacitance. A few kinds of non-polar capacitors, each having a large capacitance, are available. In particular, only a few kinds of non-polar capacitors, each having a capacitance of tens of microfarads (.mu.F) are available. Hence, as is well known, polar capacitors are modified to be non-polar and used in most cases.
FIG. 5 shows a conventional analog signal transmission circuit having two polar capacitors 8, not a non-polar capacitor. The two polar capacitors 8 are connected in serial and are non-polarized. They are connected at their negative-side terminals. They may be connected at their positive-side terminals.
FIG. 6 shows a conventional analog signal transmission circuit which differs from the circuit shown in FIG. 5 in that the transformer 6 is connected between the capacitors 8. However, the circuit has the same advantage as the circuit shown in FIG. 5.
The circuits shown in FIGS. 5 and 6, having non-polarized capacitors, however, have the following disadvantages:
One of the capacitors cannot perform its inherent function since a reverse DC voltage is applied to it. Thus, it cannot cut a DC offset current reliably. To be more specific, its capacitance decreases when a reverse voltage is applied to the capacitor. Thus, it is small, as compared with the case where a forward voltage is applied to the capacitor.